1. Field of the Invention
The present invention relates to an improved stencil or mask for use in applying solder paste on to circuit boards and to a support frame therefor.
2. Description of Related Art
It is known to produce electric circuit boards with solder connections for components, such as integrated circuit chips, by applying a solder paste to the circuit board by means very similar to screen printing. In one such known printing arrangement, a solder stencil or mask comprises a thin but still relatively stiff sheet of stainless steel or brass in which fine lines or apertures for the passage of solder paste have been formed, for example, by etching or laser cutting. The metal sheet is bonded to and supported by a thin peripherally surrounding flexible perforate sheet, of polypropylene or like, which in turn is bonded at its edges to a rectangular rigid metal frame. Such known arrangement is expensive and time-consuming to produce--particularly in view of the curing time necessary for the bonding of the adhesive. Additionally, the mesh and mask are not removably replaceable on the rectangular support frame without destruction of the mesh.
More recently it has been proposed to provide a rigid rectangular support frame wherein on two opposite sides and extending along a substantial part of the length thereof there are mechanical coupling means for interconnection with opposite sides of a metal printing stencil or mask with one mechanical coupling means being mounted on displaceable support means which are displaceable to tension the sheet when connected between the coupling means by suitable hydraulic, pneumatic or spring or mechanical tension devices as will be apparent to persons skilled in the art and well known. The mechanical coupling means includes two metal strips which underlie the normally lowermost surface of the mask sheet when in the operative position of the printing frame and mask and are coupled to the mask sheet along two opposite edges by means of a plurality of apertures in each strip at relatively large spacing and secured by screws, rivets or the like. While such arrangement is suitable for relatively small circuit boards, it does have the disadvantage of not permitting circuit boards of large area to be printed because of the projection of the opposite side strips of the mechanical coupling means beneath the lowermost surface of the mask. Additionally, the arrangement produces a very rigid mask when tensioned which may not allow for slight irregularities in the circuit boards which were tolerated by the first abovementioned arrangement.